package com.example.sale.utils;

import java.math.BigInteger;

public class SnowflakeIdGenerator {

    // 起始的时间戳（2021-01-01）
    private final static long START_TIMESTAMP = 1609459200000L;

    // 每一部分占用的位数
    private final static long SEQUENCE_BIT = 12; // 序列号占用的位数
    private final static long MACHINE_BIT = 10;  // 机器标识占用的位数

    // 每一部分的最大值
    private final static long MAX_SEQUENCE = ~(-1L << SEQUENCE_BIT);
    private final static long MAX_MACHINE_NUM = ~(-1L << MACHINE_BIT);

    // 每一部分向左的位移
    private final static long MACHINE_LEFT = SEQUENCE_BIT;
    private final static long TIMESTMP_LEFT = SEQUENCE_BIT + MACHINE_BIT;

    private long machineId; // 机器ID
    private long sequence = 0L; // 序列号
    private long lastTimestamp = -1L; // 上一次时间戳

    public SnowflakeIdGenerator(long machineId) {
        if (machineId > MAX_MACHINE_NUM || machineId < 0) {
            throw new IllegalArgumentException("machineId can't be greater than MAX_MACHINE_NUM or less than 0");
        }
        this.machineId = machineId;
    }

   public BigInteger nextBigIntegerId(){
        long id = nextId();
        return BigInteger.valueOf(id);
    }

    /**
     * 产生下一个ID
     */
    public synchronized long nextId() {
        long currTimestamp = getNewTimestamp();

        if (currTimestamp < lastTimestamp) {
            throw new RuntimeException("Clock moved backwards. Refusing to generate id");
        }

        if (currTimestamp == lastTimestamp) {
            // 相同毫秒内，序列号自增
            sequence = (sequence + 1) & MAX_SEQUENCE;
            // 同一毫秒的序列数已经达到最大
            if (sequence == 0L) {
                currTimestamp = getNextMill();
            }
        } else {
            // 不同毫秒内，序列号置为0
            sequence = 0L;
        }

        lastTimestamp = currTimestamp;

        return (currTimestamp - START_TIMESTAMP) << TIMESTMP_LEFT // 时间戳部分
                | machineId << MACHINE_LEFT // 机器标识部分
                | sequence; // 序列号部分
    }

    private long getNextMill() {
        long mill = getNewTimestamp();
        while (mill <= lastTimestamp) {
            mill = getNewTimestamp();
        }
        return mill;
    }

    private long getNewTimestamp() {
        return System.currentTimeMillis();
    }

    public static void main(String[] args) {
        SnowflakeIdGenerator idGenerator = new SnowflakeIdGenerator(1);

        for (int i = 0; i < 10; i++) {
            long id = idGenerator.nextId();
            System.out.println(id);
        }
    }
}